Circuit for generating a dynamic focusing voltage for a picture tube

ABSTRACT

In a television receiver, a dynamic, parabolic correction voltage for focusing is generated by means of a transformer whose primary winding is fed with the line deflection current or a line-frequency voltage and to whose secondary winding a capacitor is connected, which capacitor integrates the sawtooth-waveform current in the secondary winding into a parabolic voltage. New types of picture tubes require a correction voltage which does not have a parabolic profile, rather whose form corresponds approximately to the cross section through the centre of a bathtub.  
     It is an object to provide a simple circuit for generating such a bathtub-waveform correction voltage.  
     This is achieved by virtue of the fact that a frequency-dependent network, in particular a series resonant circuit comprising an inductance (L) and a capacitance (C), is connected in parallel with the capacitor (Co), which network is tuned in such a way that the network effects an additional sinusoidal current (IL/C) with a period duration approximately equal to the line trace period (TH).  
     The circuit is particularly simple, requires no active elements, effects a low power loss, can be used with a current transformer and furthermore enables blanking of the correction voltage during the vertical blanking interval.

[0001] The invention is based on a circuit for generating a dynamicfocusing voltage for a picture tube in accordance with the preamble ofclaim 1. A circuit of this type is disclosed by WO 93/06688.

[0002] In a television receiver, a high focusing DC voltage of themagnitude of 7 kV is applied to the focusing electrode of the picturetube, which voltage effects extensive focusing of the luminous spotgenerated by the electron beam on the screen over the entire screenarea.

[0003] In new types of cathode ray tubes having larger deflection anglesor screens, the change in the value—required for optimum focusing—of thefocusing voltage in the horizontal direction and in the verticaldirection over the screen area is so large that adequate focusing overthe entire screen area is not possible with a fixed focusing DC voltage.The focusing voltage must then change dynamically in accordance with therespective position of the luminous spot on the screen in the horizontaldirection and in the vertical direction.

[0004] For this purpose, it is known to add a horizontal-frequencyand/or vertical-frequency dynamic correction voltage via AC voltagecoupling to the static focusing DC voltage. The focusing voltageeffective on the focusing electrode then comprises a high DC voltagecomponent and an AC voltage component in the form of the abovementioneddynamic focusing voltage. Good matching of the respective focusingvoltage to the optimum values for focusing can be achieved with aparabolic correction voltage in the horizontal direction and in thevertical direction.

[0005] For the circuit for generating a line-frequency, paraboliccorrection voltage, there are, in principle, three different circuittypes:

[0006] In the case of a first circuit type, the primary winding of thetransformer operating as a current transformer is connected in serieswith the deflection current, in particular in series with the deflectioncoil and the so-called S capacitor or tangent capacitor. Asawtooth-waveform current flows in the secondary winding on account ofthe sawtooth-waveform deflection current in the primary winding, and isintegrated into a parabolic voltage by the capacitor.

[0007] In the case of a second circuit type, the transformer serves as avoltage transformer, the voltage across the S capacitor or so-calledtangent capacitor being fed to the primary winding.

[0008] In the case of a third circuit type, described in WO 93/06688,the primary winding of the transformer is fed with the line flybackpulse, and the transformer is deliberately designed with loose couplingbetween primary winding and secondary winding. This loose coupling hasthe following purpose: in order to convert the flyback pulse into aparabolic voltage, the flyback pulse has to be integrated twice. Thefirst integration converts the flyback pulse into a sawtooth-waveformcurrent, while the second integration integrates the sawtooth-waveformcurrent into the parabolic voltage. In this case, the inductancerequired for the first integration in the series path is formed by thedeliberately increased leakage inductance of the transformer.

[0009] In the very latest picture tubes, which operate for example witha so-called Eureka gun, also called MR gun, adequate focusing can nolonger be achieved with a parabolic correction voltage. It has beenshown that, on account of the special gun optical system and the largeradius of curvature of the screen of these tubes, a correction voltagewhich is less pointed and wider at the vertex than a parabola isadvantageous. This new form of correction voltage approximatelycorresponds to the cross section through the centre of a commerciallyavailable bathtub and is therefore designated hereinafter bybathtub-waveform correction voltage. A correction voltage of this typecan no longer be generated by simple integration of a sawtooth-waveformcurrent.

[0010] It is desirable, therefore, to provide a simple circuit withwhich the abovementioned bathtub-waveform horizontal-frequencycorrection voltage can be generated from a line-frequency current or aline-frequency voltage. The invention specified in claim 1 is suitablefor this purpose. Advantageous embodiments and developments of theinvention are specified in the subclaims.

[0011] The invention can be applied to the first and third circuit typesdescribed. In the case of the invention, in order to generate thebathtub-waveform correction voltage, a frequency-dependent network isprovided in parallel with the capacitor across the secondary winding ofthe transformer supplying the correction voltage, which network is tunedin such a way that it effects an additional sinusoidal current with aperiod duration approximately equal to the line trace period.

[0012] The circuit according to the invention is based on the followingeffect. The secondary winding of the transformer acts as a currentsource which effects a sawtooth-waveform current in the combinationcomprising the capacitor and the abovementioned network. The resultingcurrent in the capacitor is the difference between the sawtooth-waveformcurrent from the secondary winding and the network. The integration ofthe current by the capacitor produces a voltage having the desiredbathtub-waveform profile.

[0013] The invention has a number of advantages. The circuit isparticularly simple since, for example in a preferred embodiment, itonly requires two passive components in the form of a capacitor and acoil, and no active elements. As a result, the power loss is alsominimized. The invention is particularly suitable for converting, in asimple manner, a known circuit for generating a parabolic correctionvoltage of the abovementioned first and third circuit types into acircuit for generating a bathtub-waveform correction voltage. Moreover,the circuit according to the invention does not require additionaladjustment of components. Furthermore, in the case of the third circuittype described, the circuit makes it possible, in a simple manner, toeffect blanking of the correction voltage during the vertical blankinginterval.

[0014] The period duration of the resonant frequency of the network neednot correspond exactly to the line trace period, but rather may, inparticular, be somewhat longer than the line trace period. Therespectively required form of the correction voltage can be achieved bycorresponding dimensioning of the impedance of the network whilstadhering to the abovementioned tuning. In the case of a network in theform of an L/C series resonant circuit, smaller values for thecapacitance and higher values for the conductance of the series circuitreduce the current in the series circuit and give the correction voltagegenerated a more parabolic profile. The maximum curve shaping in thedirection of the bathtub shape is achieved if the peak value of thesinusoidal current in the series circuit reaches half the peak value ofthe current supplied by the secondary winding. Higher peak currents inthe series circuit can lead to visibility of the oscillation frequencyin the correction voltage on the screen. The respectively requiredamplitude of the correction voltage can be set by means of the turnsratio and, in the case of the third circuit type, by means of the airgap of the transformer.

[0015] In the dimensioning of the circuit according to the invention,under certain conditions the following difficulty may arise: in order togenerate a dynamic correction voltage with an amplitude in the region of1 kV, the capacitor connected in parallel with the secondary windingmust have a value of the order of magnitude of a few hundred pF in thecase of the abovementioned first and third circuit types, because only asmall current having a peak value of about 10 mA flows in the secondarywinding. Since the current in the LC series circuit must be less than orequal to half the sawtooth current, the result is a low capacitance ofless than 500 pF for the capacitor and a relatively high value of morethan 20 mH for the inductance. Such a high inductance is difficult torealize in practice. In order to eliminate this difficulty, inaccordance with one development of the invention, the transformer isprovided with a third winding, and the network, in particular the L/Cseries resonant circuit, is connected in parallel with this thirdwinding. The third winding is preferably fixedly coupled to thesecondary winding. Given a turns ratio of the secondary winding to thethird winding of the order of magnitude of 6, in the case of a shortcircuit, the current in this third winding is six times higher than thecurrent in the secondary winding W2, and the transformed capacitance ofthe capacitor across the third winding W3 is six times greater than thecapacitance of the capacitor itself. With the low output impedance ofthe third winding, the embodiment of a corresponding L/C series resonantcircuit is significantly simpler. For deflection at twice the linefrequency, so-called 2H deflection, values of 6 nF for C and 2 mH for Lcan be used for the L/C series resonant circuit. This development of theinvention can also advantageously be applied to the abovementioned thirdcircuit type.

[0016] The function of the circuit—disclosed in WO 9306688—for blankingthe correction voltage during the vertical flyback period is notadversely affected by the additional L/C resonant circuit according tothe invention.

[0017] The invention is explained below with reference to the drawingusing a plurality of exemplary embodiments. In the drawing:

[0018]FIG. 1 shows an exemplary embodiment of the invention for theabovementioned first circuit type with a current transformer,

[0019]FIG. 2 shows an exemplary embodiment for the abovementioned thirdcircuit type,

[0020]FIG. 3 shows a development of the circuit as shown in FIG. 2 foradditional blanking of the correction voltage during the verticalflyback period,

[0021]FIG. 4 shows curved profiles for currents and voltages in thecircuits illustrated, and

[0022]FIG. 5 shows various empirically determined and calculatedprofiles of correction voltages.

[0023]FIG. 1 shows part of a line deflection circuit for a picture tubeB in a television receiver. The S-shaping or tangent capacitor Cs andthe primary winding W1 of a current transformer Tr lie in the path ofthe line deflection current iH which flows through the horizontaldeflection coil of the picture tube B. A capacitor Co (Co=Coutput) isconnected in parallel with the secondary winding W2 of the transformerTr, the series circuit comprising an inductance L and a capacitance Cbeing connected in parallel with the said capacitor. The line deflectioncurrent iH initially generates a sawtooth-waveform current in thesecondary winding W2, this sawtooth-waveform current being integratedinto a parabolic voltage by the capacitor Co. The resonant frequency ofthe series resonant circuit L/C is tuned to the line trace period of 52μs. The series resonant circuit L/C generates an additional sinusoidalcurrent in the capacitor Co, which current converts the inherentlyparabolic voltage across Co into a bathtub-waveform correction voltageUk. This correction voltage Uk is fed to the circuit point a via ACvoltage coupling with the coupling capacitor Ck, and, at the saidcircuit point a, is combined with the static focusing DC voltage Ufs andfed as total focusing voltage Ufocus to the focusing electrode F of thepicture tube B. Ri designates the internal resistance of the source forUfs. The period duration of the resonant frequency of the seriesresonant circuit L/C may also be somewhat longer than the line traceperiod.

[0024]FIG. 2 shows an embodiment for the abovementioned third circuittype. In this case, the transformer Tr acts as a current transformer,the line flyback pulse FB being fed to the primary winding W1. Thetransformer Tr has particularly loose coupling between the primarywinding W1 and the secondary winding W2, with the result that arelatively large leakage inductance takes effect, which is connected inseries with the windings W1 and W2 in the equivalent circuit diagram.This leakage inductance is utilized such that a double integration takesplace. The first integration integrates the flyback pulse FB into asawtooth-waveform current, and the second integration converts thesawtooth-waveform current in the secondary winding W2 into a parabolicvoltage. From the latter, as in FIG. 1, the series resonant circuit L/Cthen generates the bathtub-waveform correction voltage Uk, since theentire sawtooth-waveform current from W2 does not flow through thecapacitor Co, but rather only its difference with respect to thesinusoidal current in L/C.

[0025]FIG. 3 shows an exemplary embodiment for a development of theinvention with additional blanking of the correction voltage Uk duringthe vertical blanking interval, which likewise operates in accordancewith the third circuit type or FIG. 2. The voltage transformer Trcontains a third winding W3, and the series resonant circuit L/C isconnected in parallel with this third winding W3. The turns ratio W2/W3is of the order of magnitude of 6 and may also be 5 or 7. First of all,this solution avoids the difficulty described in the introduction in thecase of the dimensioning of the series resonant circuit L/C. Connectedin parallel with the third winding W3 is a bridge rectifier G, to whoseoutput a transistor T1 is connected. Connected in parallel with thecollector/emitter path of the transistor T1 is a charging capacitor C1with current limiting Ri, and a vertical blanking pulse V is fed via thevoltage divider R2/R3 to the base of the transistor T1. During thevertical trace period, the transistor T1 is turned off, the rectifier Gis not loaded, and the circuit operates in a similar manner to that inFIG. 2. During the vertical blanking interval, the transistor T1 isturned on by the vertical blanking pulse V. The rectifier G isshort-circuited, with the result that the winding W3 is likewiseshort-circuited and the correction voltage Uk is blanked. The blankingof the correction voltage Uk during the vertical blanking interval isnecessary for the following reason: the high AC voltage in the form ofthe correction voltage Uk on the focusing electrode F of the picturetube B produces capacitive feedback to the cathodes and the grid 2 ofthe picture tube B. This influences the so-called cut-off measuring,that is to say the measurement of the beam current for the black value,of the three guns of the picture tube B, which can lead to changes inthe colour temperature and to disturbances in the cut-off control.

[0026]FIG. 4 shows, for the line trace period TH of 52 μs, thesawtooth-waveform current IW2 through the secondary winding W2, thesinusoidal current IL/C through the series resonant circuit L/C and thecurrent Ico effected thereby—with the period of the line trace period THthrough the capacitor Co. This current generates the requiredbathtub-waveform correction voltage at Co in the manner described. Thecurrent waveforms illustrated relate to the embodiments shown in FIG. 1and FIG. 2. IW2 and Ico are identical in the case of the embodimentaccording to FIG. 3.

[0027]FIG. 5 shows various required and calculated profiles of thebathtub-waveform correction voltage Uk in comparison with a paraboliccorrection voltage Upar. Uco designates the voltage across the capacitorCo, UG designates the required correction voltage for the colour greenand the curve URB designates the required correction voltage for thecolours red and blue. The last voltages are approximately identicalowing to the symmetrical position of the electron guns for red and bluerelative to the centre, while the correction voltage UG deviatestherefrom owing to the position of the electron gun for green in thecentre of the inline arrangement of the three guns. In practice, only acorrection voltage which approximately corresponds to the averageprofile of the curves UG and URB is used.

LIST OF REFERENCE SYMBOLS

[0028] a Circuit point

[0029] B Picture tube

[0030] C Capacitor

[0031] C1 Capacitor

[0032] Ck Coupling capacitor

[0033] Co Capacitor

[0034] Cs Tangent capacitor

[0035] FB Line flyback pulse

[0036] G Bridge rectifier

[0037] Ico Current

[0038] IL/C Current

[0039] IW2 Current

[0040] L Coil

[0041] R1 Resistor

[0042] R2 Resistor

[0043] R3 Resistor

[0044] Ri Internal resistor

[0045] T1 Transistor

[0046] TH Line trace period

[0047] Tr Transformer

[0048] Uco Voltage

[0049] Ufocus Focusing voltage

[0050] Ufs Static focusing voltage

[0051] UG Voltage

[0052] Uk Correction voltage

[0053] Upar Correction voltage

[0054] URB Voltage

[0055] W1 Primary winding

[0056] W2 Secondary winding

[0057] W3 Third winding

1. Circuit for generating a dynamic focusing voltage (Uk) for a picturetube (B) by means of a transformer (Tr), to whose primary winding (W1) aline-frequency current or a line-frequency voltage is fed and with whosesecondary winding (W2) a capacitor (Co) is connected in parallel,characterized in that, in order to generate a bathtub-waveformcorrection voltage (Uk) deviating from the parabolic form, afrequency-dependent network (L,C) is connected in parallel with thecapacitor (Co), the said network being tuned in such a way that thenetwork effects an additional sinusoidal current (IL/C) with a periodduration approximately equal to the line trace period (TH).
 2. Circuitaccording to claim 1, characterized in that the period duration of thesinusoidal current (IL/C) is somewhat longer than the line trace period(TH).
 3. Circuit according to claim 1, characterized in that the networkis formed by the series circuit comprising an inductance (L) and acapacitance (C).
 4. Circuit according to claim 1, characterized in thatthe required form of the correction voltage (Uk) is set by the impedanceof the network (L, C).
 5. Circuit according to claim 1, characterized inthat the required amplitude of the required correction voltage (Uk) isset by the turns ratio and/or the air gap of the transformer (Tr). 6.Circuit according to claim 1, characterized in that the transformer hasa third winding (W3) and the network (L, C) is connected in parallelwith this third winding (W3).
 7. Circuit according to claim 5,characterized in that the third winding (W3) is fixedly coupled to thesecondary winding (W2).
 8. Circuit according to claim 7, characterizedin that the turns ratio of the secondary winding (W2) to the thirdwinding (W3) is of the order of magnitude of 6.